Control apparatus for two-speed compressor

ABSTRACT

The disclosure describes a refrigerant compressor of the type driven by a motor capable of operating at a low speed and a high speed, the motor including first, second and third stator coils. First, second and third temperature sensors are located adjacent the first, second and third stator coils, respectively, for detecting any change in temperature of the coils. If the temperature of any of the stator coils exceeds a predetermined value, a control device operates a switch in a pilot circuit that turns off the motor before damage from excessive heat can occur. A unique contactor assembly utilizing both electrical and mechanical interlocks interconnects the stator coils to provide two speed operation.

This is a division of application Ser. No. 436,179 filed Jan. 24, 1974,now U.S. Pat. No. 3,935,519.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to refrigerant compressors driven by anelectrical motor, and more particularly relates to means for controllinga refrigerant compressor utilizing a two-speed electrical motor.

Refrigerant compressors utilizing a two-speed compressor motor have beendevised in the past. One such arrangement is shown in U.S. Pat. No.3,584,980 (Cawley et al -- June 15, 1971). Although a two speedcompressor results in improved performance, the applicants havediscovered that such an arrangement requires precise control in order toprevent damage to the compressor motor during two speed operation.

Compressor motors adapted to operate from a three phase source ofelectrical power generally comprise at least one stator coil for each ofthe electrical phases. The applicants have found that each of thesecoils must be protected from thermal damage in order to ensure longmotor life. Since thermal damage can occur rapidly under certain adverseoperating conditions, the coils must be protected by sensitivetemperature detectors capable of rapid response.

Although one temperature sensor placed adjacent one of the coils willtend to provide protection against thermal destruction due to a lockedrotor or running overload, a single sensor or dual sensors will not giveadequate primary and secondary single phasing protection. The applicantshave discovered that protection is enhanced by providing a sensor foreach of the phase windings or coils in a multiphase motor. These sensorsprovide an indication of the temperature adjacent each coil associatedwith a different phase of electrical power. If one of the sensorsindicates an abnormally high temperature in an associated stator coil,control means operate a switch in a motor pilot circuit in order toremove all electrical power from the motor. In this manner, theapparatus provides primary and secondary single phasing protection,along with locked rotor and running overload protection.

Accordingly, it is an object of the present invention to provide arefrigerant compressor in which a motor adapted to operate on multiphaseelectrical power has a temperature sensor located adjacent each motorcoil associated with a different phase of the electrical power, so thateach coil in the motor is adequately protected against thermal damage.

It is still another object of the invention to provide control apparatusof the foregoing type in which a control device interrupts power to themotor if any one of the sensors detects an abnormally high temperaturein any of the motor coils.

The applicants have also discovered that the control of a two-speedcompressor motor requires careful design of the contactors used toswitch from a high speed mode of operation to a low speed mode ofoperation and vice versa. In a compressor motor adapted to be operatedfrom multi-phase electrical power, multiple stator coils are frequentlyprovided and must be interconnected in different ways in order toachieve different speeds of operation.

The applicants have discovered that the interconnection of the coilsmust be achieved prior to the application of electrical power to thecoils in order to avoid power surges and thermal damage to the motor. Inaddition, the applicants have invented a unique system of electricallyand mechanically interlocking the contactors required for two-speedoperation in order to assure a failsafe system of operation.

Accordingly, it is another object of the present invention to providecontrol apparatus for a two-speed compressor motor in which the coils inthe motor are interconnected before any power is applied to the coils.

It is still another object of the present invention to provide motorcontrol apparatus of the foregoing type in which both mechanicalelectrical interlocks are operated by and simultaneously with thecontactors controlling the two-speed operation of the motor.

DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the presentinvention will hereafter appear in connection with the accompanyingdrawings wherein:

FIG. 1 is an isometric, fragmentary, partially cross-sectional view of arefrigerant compressor and motor embodying control apparatus made inaccordance with the present invention;

FIG. 2 is an electrical schematic diagram showing a preferred form ofpilot circuit and contactor control circuit made in accordance with thepresent invention;

FIG. 3 is a fragmentary electrical schematic diagram showing the mannerin which the stator coils shown in FIG. 2 are interconnected for highspeed operation;

FIG. 4 is a fragmentary electrical schematic diagram showing the mannerin which the stator coils shown in FIG. 2 are interconnected for lowspeed operation;

FIG. 5 is an electrical schematic block diagram of the motor protectionmodule illustrated in FIG. 2;

FIG. 6 is a top plan view of a preferred form of contact or made inaccordance with the present invention; and

FIG. 7 is a cross-sectional view taken along line 7--7 in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a refrigerant compressor 10embodying a preferred form of the present invention. Compressor 10comprises a sealed outer casing 11 which includes an upper shell 12(having a top surface 13) that is welded to a lower shell 14. Aplurality of legs 16 are suitably secured to compressor 10 to support itin an upright position.

Resiliently supported within the outer casing by resilient coil springmeans 20 is a compression mechanism 22. The compression mechanismcomprises a compressor block 24 which defines a cylinder 26, togetherwith additional cylinder (not shown). A movable piston 28 reciprocateswithin cylinder 26 in order to compress a refrigerant vapor. Additionalpistons, like piston 28, reciprocate in the additional cylinders, notshown. Each of the pistons is driven from a vertically-disposed driveshaft 30. The lower portion of the outer casing forms an oil sump inwhich the oil level is visible through an oil sight glass 31.

An electrical motor 32 is used to drive compression mechanism 22. Themotor comprises a stator 34 which includes windings 36. A rotatablerotor 38 is inductively coupled to stator 34 and is mechanically coupledto drive shaft 30.

Provided at the end of each cylinder, including cylinder 26, and closingthe end of each cylinder cavity, is a valve assembly, such as a valveassembly 40. The valve assembly includes a discharge valve and a suctionvalve. The suction valve opens on the suction stroke of piston 28 topermit refrigerant gas (suction gas) to enter cylinder 26 through asuction line 46. On the compression stroke of piston 28, the suctionvalve closes and the discharge valve opens to permit the flow ofcompressed refrigerant gas to a discharge muffler in the compressionmechanism and then to discharge lines 48. The compressed gas istransmitted through the discharge lines to a conventional condensor (notshown).

A thermal density sensor 50 is located inside outer casing 11. Thesensor comprises a thermostat which is screwed into compressor block 24in the position shown in FIG. 1. The thermostat is preferably the sametype of temperature limiting device shown in U.S. Pat. No. 3,278,111(Parker -- Oct. 11, 1966) in which switch contacts 58 (FIG. 2) and atemperature responsive element are contained within a single housing.Sensor 50 is set to open electrical contacts 58 at about 22° F., plus orminus 3° F., and to close contacts 58 at about 32° F., plus or minus 3°F. Contacts 58 are electrically connected to sensor 50 by conductors 54and 55. Sensor 50 prevents the start up of motor 32 when the temperatureof the running parts of the compression mechanism is below apredetermined value which inhibits adequate lubrication.

As shown in FIG. 1, a control box assembly 62, which comprises a top 64and sidewalls 66-69, is physically coupled to top surface 13 of outercasing 11. An electrical control assembly 72, contained within controlbox 62, includes a contactor 74, as well as compressor terminals.

Control assembly 72 also comprises a high pressure switch assembly 82located in the discharge gas spud on the exterior of the outer casing.Switch contacts 84 (FIG. 2) are contained within assembly 82.

The control assembly also includes a low pressure switch assembly 90that incorporates switch contacts 92 (FIG. 2). The switch contacts areconnected in the motor pilot circuit as shown in FIG. 2.

Control assembly 72 further comprises a discharge gas temperature sensor100 which is the same type of device as sensor 50 described above.Sensor 100 includes a set of switch contacts 102 (FIG. 2) which areconnected to the sensor by conductors 104 and 105. The discharge gastemperature sensor senses discharge gas temperature at its source so asto terminate compressor motor operation if the discharge gas temperatureexceeds a predetermined high value (on the order of 300° F.) to preventbreakdown of the compressor oil and damage to the running parts of thecompression mechanism.

In operation, each of the switches 58, 84, 92 and 102 shown in FIG. 2must be in a closed circuit condition in order for motor 32 to beoperated. If the temperature of the running parts of the compressionassembly, for example, the drive shaft, piston, rods, and wrist pin, istoo low, thermal density sensor 50 opens switch contacts 58, therebypreventing motor 32 from starting up and damaging the compressionmechanism. If high pressure switch assembly 82 detects pressure aboveapproximately 410 PSIG in discharge lines 48, switch contacts 84 areopen circuited or opened to stop motor 32. Likewise, if a temperatureabove approximately 300° E. in the discharge manifold is detected bydischarge gas temperature sensor 100, switch contacts 102 are opened tostop motor 32. In a similar manner, if low pressure switch assembly 90detects pressure below approximately 25 PSIG inside outer casing 11,switch contacts 92 are opened in order to stop motor 32.

Referring to FIGS. 1 and 2, three phase electrical power is supplied tostator coils 36 through a shielded cable 106 and over conductors108-110. Stator coils 36 comprise inner coils 112-114 which are joinedat a common end point 116 and which also are provided with terminals118-120, respectively. The stator coils further includes outer coils122-124 which terminate in terminals 128-130, respectively. Each ofinner coils 112-114 and outer coils 122-124 is wired to accept adifferent phase of electrical power from three phase power conductors108-110, respectively.

Referring to FIG. 2, electrical control assembly 72 also comprises atying contactor 134 which is used to interconnect stator coils 36 forhigh speed operation when energized. The tying contactor comprises aswitch arm T136 which movably operates between terminals 138, 139 and aswitch arm T140 which movably operates between terminals 142 and 143.

In order to operate motor 32 at the high speed, a high speed contactor146 is provided. As shown in FIG. 2, the contactor includes a switch armH150 that movably operates between terminals 152, 153, a switch arm H156which movably operates between terminals 158, 159, and a switch arm H162which movably operates between terminals 164 and 165.

Referring to FIGS. 2 and 6, a low speed contactor 170 is used to operatemotor 32 at low speed. The contactor comprises a body 171, a switch armL172 which movably operates between terminals 174, 175, a switch armL178 that movably operates between terminals 180, 181, and a switch armL184 that movably operates between terminals 186 and 187.

Each of the above described switch arms T136, T140, H150, H156, H162,L172, L178 and L184 is normally open in the manner shown in FIG. 2.

Control assembly 72 also includes a pilot control circuit 188 that has alow speed branch 189 for operating motor 32 at low speed and a highspeed branch 190 for operating motor 32 at high speed.

Within pilot control circuit 188 is included an electrical interlockassembly 191. Referring to FIG. 2, the electrical interlock assemblycomprises a normally-closed tying interlock switch 192 having a switcharm T194 that is movably operated between terminals 196 and 197. Anormally open tying interlock switch 200 includes a switch arm T202 thatis movably operated between terminals 204 and 205. A normally closedhigh-speed interlock switch 208 includes a switch arm H210 that ismovably operated between terminals 212 and 213, and a normally closedlow speed interlock switch 216 includes a switch arm L218 that ismovably operated between terminals 220 and 221. As indicated in FIG. 2,tying interlock switch arms T194 and T202 are physically ganged to tyingcontactor switch arms T136 and T140. Likewise, switch arm H210 isphysically ganged to high speed contactor switch arms H150, H156 andH162, and low speed interlock switch arm L218 is physically ganged tolow speed contactor switch arms L172, L178 and L184.

Referring to FIG. 6, interlocking of the contactors is also achieved bya mechanical interlock assembly 224. The tying contactor switch arms, aswell as tying interlock switch arms T194 and T202 are each operated by atying plunger 226 that is lowered by an arm 228 rotatably mounted on apivot rod 230. A coil spring 232 biases the plunger in an upwarddirection as shown in FIG. 7. An interlock finger 234 having an uppersurface 236 and a side face 238 extends above pivot rod 230 in order tointerlock the tying contactor and the low speed contactor.

The mechanical interlock assembly also includes a low speed plunger 242which operates the low speed contactor switch arms, as well as interlockswitch arm L218. The plunger is operated in a vertical direction by anarm 244 that is rotatably mounted to a pivot rod 246. Arm 244 terminatesin opposed prongs 245a, 245b into which an extending finger 247 isfitted. Finger 247 is integrally formed with plunger 242. The plunger isbiased in an upward direction (as shown in FIG. 6) by a coil spring 248.An interlock finger 250 having an upper surface 252 and a side face 254extends above pivot rod 246 in order to cooperate with interlock finger234.

Referring to FIG. 2, the pilot control circuit also includes a conductor262 which conducts AC current to a speed switch arm 264 that is movablebetween a low speed terminal 266 and a high speed terminal 268. The lowspeed branch of the circuit includes a conventional 2 second time delaydevice 273 as well as a low speed relay coil 278. The high speed branchof the circuit includes a tying relay coil 274 and a high speed relaycoil 276 connected as shown.

In order to operate the control assembly, conductors 108-110 areconnected to a three phase source of AC voltage, and speed switch arm264 is moved to the desired speed terminal. Assuming switch arm 264 isplaced in contact with high speed terminal 268 and that current isconducted through return conductor 279 by switch contacts 58, 84, 92 and102, the normally-closed switch arm L218 conducts current through tyingrelay coil 274. As soon as relay coil 274 is energized, plunger 226 islowered so that switch arms T136 and T140 are closed, interlock switcharm T194 is opened, and interlock switch arm T202 is closed. This is animportant feature, since it interconnects stator coils 36 for high speedoperation before power is applied to the coils. This mode of operationprevents transient or circulating currents which could be caused by theapplication of power before the tying of and interconnecting of thecoils is completed. As plunger 226 moves downward, interlock finger 234moves to the position shown in phantom in which upper surface 236 movesinto the path of low speed interlock finger 250, thereby physicallypreventing low speed plunger 242 from closing low speed switch armsL172, L178, and L184 (FIGS. 6 and 7). In addition, the opening of switcharm T194 electrically prevents the energization of low speed relay coil278.

As soon as switch arm T202 is closed, high speed relay coil 276 isenergized and closes high speed switch arms H150, H156 and H162 so thatthree phase AC power is applied to the stator coils. Theinterconnections made by the energization of relay coils 274 and 276 isshown in FIG. 3. High speed interlock switch H210 arm opens, alsopreventing low speed coil 278 from being energized as did switch armT194. The high speed switch arms may be operated by a contactor similarto the one shown in FIGS. 6 and 7. As soon as these connections aremade, the motor begins to turn at its high speed rate.

Assuming that speed switch arm 264 is moved into contact with low speedterminal 266, time delay device 273 allows current to flow in low speedbranch 189 after a two second delay. Current passes through the normallyclosed switch arms H210 and T194 in order to energize low speed relaycoil 278. The energization of relay coil 278 causes low speed plunger242 to move downward (as seen in FIG. 6) so that low speed switch armsL172, L178 and L184 are closed and low speed interlock switch arm L218is opened. At the same time, upper surface 252 of interlock finger 250moves to the position shown in phantom to prevent interlock finger 234from lowering the tying plunger 226, thereby preventing tying switcharms T136 and T140 from closing. As a result, relay coils 274 and 276are physically prevented from being energized. In addition, the openingof switch arm L218 electrically prevents the relay coils from beingenergized. As soon as the low speed switch arms in low speed contactor170 are closed, the connections shown in FIG. 4 are completed so thatmotor 32 rotates at its low rate of speed.

Referring to FIGS. 2 and 5, stator coils 236 are protected fromexcessive temperatures by a motor protection assembly 282. The assemblyincludes sensors 284-286 that are preferably embedded in stator coils112-114, respectively. If the sensors cannot be embedded in stator coils112-114, they may be placed adjacent to coils 112-114 but in definitethermal contact. Applicants have found that contact between sensors andcoils 122-124 only will not provide satisfactory protection fromoverheat should the tie contactor 134 fail in the open position whilecompressor 10 is operating at high speed. The sensors have a positive ornegative temperatures coefficient of resistance so that, as the statorcoils heat up, the resistance of the sensors changes. The resistancechange with temperature may be linear or non-linear.

The sensors are used to provide temperature information for a solidstate motor protection module 288. The motor protection module may be asolid state device, such as a Robert Shaw model MP23 "Statorstal MotorProtector". This device is shown in more detail in FIG. 5.

Module 288 includes a bridge resistor 290 that is used as one leg of abridge, the other leg comprising the sensors 284-286. The sensorresistors and bridge resistor are wired together in a bridgeconfiguration which controls an OR gate 292. If the reference point 293at the junction between the bridge resistor and the sensors is negative,a current flows through OR circuit 292 to energize a semiconductor relaycircuit 294. Power is supplied to the relay circuit through atransformer 296 and single phase power conductor 262 and 298. Relaycircuit 294 controls a triac 300 that is connected to pilot circuitconductor 297 over conductors 302 and 303. Relay circuit 294 alsocontrols a differential switch 304 and a differential resistor 306 thatcan be switched into the bridge circuit.

During normal running conditions, the resistance of sensors 284-286 islower than the resistance of bridge resistor 290. In this case, thebridge is unbalanced and reference point 293 is positive so that nocurrent flows through OR circuit 292. Under these conditions, relaycircuit 294 is energized so that triac 300 is switched to its conductivestate. In its conductive state, triac 300 enables current to flowthrough conductor 279 in the pilot circuit so that relay coils 274, 276and 278 may be energized. When relay circuit 294 is energized,differential switch 304 is closed and differential resistor 306 isshorted out of the bridge circuit comprising sensors 284-286 and bridgeresistor 290.

As the stator coils heat up, sensors 284-286 also heat up and theirresistance increases. If the sensor resistance exceeds the module cutout value, the bridge becomes balanced so that reference point 293 isswitched to a negative polarity and current flows through OR circuit 292in order to de-energize semi-conductor relay circuit 294. As soon asrelay circuit 294 is de-energized, triac 300 is switched to itsnon-conductive state so that current can no longer flow throughconductor 279. As a result, the pilot control circuit returns each ofcontactors 134, 146 and 170 to its open circuit condition so that themotor is turned off. At the same time relay circuit 294 is de-energized,differential switch 304 opens so that differential resistor 306 isconnected in series with sensors 284-286. This increases the unbalanceof the bridge so that OR circuit 292 prevents current from flowing torelay circuit 294 until the resistance in the sensors drops below thecut in value.

As soon as the stator coils have cooled, the sensor temperature andresistance drop in value until the combined resistance of sensors284-286 and differential resistor 306 is less than the resistance ofbridge resistor 290. At this point in time, the bridge circuit isunbalanced in the opposite direction so that the polarity of junction293 is again switched positive. As a result, OR circuit 292 conductscurrent, relay circuit 294 is energized and triac 300 again is switchedto its conductive state. At this point in time, motor 32 again begins torotate.

Those skilled in the art will recognize that only a preferred embodimenthas been described herein and that the embodiment may be modified andaltered without departing from the true spirit and scope of theinvention as defined in the accompanying claims.

What is claimed is:
 1. In a refrigerant compressor of the type having acompression mechanism for receiving refrigerant gas from a suction line,compressing the refrigerant gas and discharging the compressedrefrigerant gas through a discharge line, and a motor for rotating shaftmeans in the compression mechanism at low speed and at high speed from asource of three-phase electrical power including first, second and thirdpower terminals, said motor including a first stator coil, a secondstator coil and a third stator coil adapted to receive three-phaseelectrical power, each stator coil having an inner end point connectedto a common conductor and also having an outer end point, improvedcontrol apparatus for protecting the stator coils comprising:a fourthstator coil having a second end point and also having a first end pointconnected to the outer end point of the first stator coil; a fifthstator coil having a second end point and also having a first end pointconnected to the outer end of the second stator coil; a sixth statorcoil having a second end point and also having a first end pointconnected to the outer end point of the third stator coil; first sensormeans located adjacent the first coil for indicating a change intemperature of the first coil; second sensor means located adjacent thesecond coil for indicating a change in temperature of the second coil;third sensor means located adjacent the third coil for indicating achange in temperature of the third coil; first control means forinterrupting the flow of current to said stator coils in response to anindication from any of said sensor means that the temperature of any ofsaid stator coils exceeds a predetermined value; second control meansfor interconnecting the second end points of the fourth, fifth and sixthstator coils, for connecting the outer end point of the first statorcoil to the first power terminal, for connecting the outer end point ofthe second stator coil to the second power terminal and for connectingthe outer end point of the third stator coil to the third powerterminal, whereby the motor rotates at high speed; and third controlmeans for connecting the second end point of the fourth stator coil tothe first power terminal, for connecting second end point of the fifthstator coil to the second power terminal and for connecting the secondend point of the sixth stator coil to the third power terminal, wherebythe motor rotates at low speed.
 2. Apparatus, as claimed in claim 1,wherein the first control means comprises:a bridge resistor; means forconnecting the bridge resistor and the first, second and third sensormeans in a bridge circuit configuration to form a junction between thebridge resistor and one of the sensor means; bias means for biasing thebridge configuration so that the junction has a first polarity when thefirst, second and third stator coils are below a predeterminedtemperature and the junction has a second polarity when any of thefirst, second or third stator coils is above a predeterminedtemperature; and means for interrupting the flow of current to all thestator coils in response to the second polarity of said junction.